Magnetic resonance may be used to analyze medical and/or chemical samples. Specifically, the diverse chemical constituents and/or the special distributions of such constituents of the sample may be analyzed through the application of the magnetic resonance phenomena. To implement the magnetic resonance phenomena, RF radiation is applied to the sample by a surrounding structure. The resulting resonant signals are picked-up for analysis by the same or a different surrounding structure. The structure surrounding the sample may be a helical coil, saddle coil, resonant cavity, or a bird cage resonator. The bird cage coil is particularly well suited to large volume samples as are routinely encountered with apparatus for medical imaging and in vivo analytic spectroscopy. Bird cage coils are discussed by Hayes et al, J. Mag. Res., vol. 63, pp. 622-628 (1985).
A bird cage coil is frequently described as a ladder circuit which closes on itself, wherein the current flow around the coil is distributed sinusoidally. It is often asserted that the bird cage coil is essentially a transmission line. As a tuned RF circuit, it is employed in nuclear magnetic resonance ("NMR") apparatus for either or both of the functions of RF excitation and signal detection.
The bird cage coil differs from saddle coil, helices and other structures by its discrete structure that employs phase shifts to provide the proper sinusoidal current distribution. For the bird cage coil, there is a requirement that the phase shift be discretely distributed around the circumference of the coil from zero to 2.pi. (or 2.pi.n, where n is an integer). The phase shift of each element is quite frequency dependent, and as a consequence, the bird cage coil is tuned at a discrete frequency to achieve the desired phase shift constraint.
As mentioned above, the bird cage structure may be regarded as a periodic structure which closes on itself. Periodic elements of the structure produce phase shifts which must aggregate to some multiple of 2.pi. when summed over the closed loop. Geometrically, the resonator has cylindrical symmetry, and it is desired that RF current in the axial direction along the periphery of the structure be proportional to sin .theta. where .theta. is the azimuthal angle about the cylindrical axis.
To facilitate the study of diverse chemical constituents of the sample, it is desirable to achieve a multiple tuned bird cage coil in order to obtain data at more than one resonant frequency, either concurrently or in separate measurements. Conventional double-tuned bird cage coils attempt to operate at two NMR frequencies, i.e., for two separate nuclei. In a spectroscopic examination, typically images acquired from a proton channel are used to identify the location of interest, and a second channel may be used to tune to the X-nuclei, such as .sup.3 He, .sup.31 P, .sup.129 Xe, .sup.23 Na or .sup.13 C, for localization spectroscopy or second nuclei imaging. Usually double tuned bird cage coils are preferred over two single tuned bird cage coils because double tuned coils do not require patient repositioning during coils switching, and the double tuned coil provides the option of proton decoupling experiments.
Because bird cage coils are highly symmetric structures, consistency of the components of the coil is essential. A small variation in the value or positioning of one of the components can destroy the uniformity of the magnetic field profile and can complicate the tuning process. Because off-the-shelf commercial inductors typically have a 5% to 10% consistency rating, most conventional double tuned bird cage coils require hand-made solenoid inductors with better consistency ratings. However, the hand-made solenoid inductors are difficult and expensive to construct. Additionally, the hand-made solenoid inductors are difficult to precisely reproduce. Because consistency of the components is essential for bird cage coils, small variation in the value or positioning of the hand-made solenoid inductors can destroy the uniformity of the magnetic field profile and can complicate the tuning process. Another shortcoming of the solenoid inductor is that it generates an intense local magnetic field near its ends. This local field can create undesired effects to the main magnetic field of the bird cage coil.
A conventional double-tuned bird cage coil that does not used lumped inductances or hand-made inductors is discussed by Murphy-Boesch et al, J. Mag. Res., vol. 103, pp. 103-114 (1994). Murphy-Boesch described using different coil lengths to provide two inductance values instead of lumped inductances, namely by providing rings with different lengths. One problem with providing different coil lengths is that the possible lengths of the ring conductor are limited by dimensional limitations, in other words, bird cages can only be so large. The dimensional limitations limit the possible range of inductance values. Additionally, the longer the conductor of the coil, the less concentrated the magnetic field resulting in limited performance. Furthermore, the differing lengths result in a less homogenous field with different field profiles for the low and high resonant frequencies.
Another shortcoming of some conventional double-tuned bird cage coils, including those described by Murphy-Boesch, is the difficulty in tuning the resonant frequencies. Conventional double-tuned bird cage coils have dependent tuning. An example of a double-tuned birdcage coil is described in U.S. Pat. No. 4,916,418, assigned to the assignee of the present invention and incorporated herein by reference. For example, when tuning the bird cage coil to the lower resonant frequency, and inductance values are fixed, adjusting one set of the capacitances on the bird cage coil not only affects the low frequency but also the high frequency. In some double-tuned bird cage coils, at least two different sets of capacitances are in the path of both the low and high frequency currents, so adjusting one set of capacitances affects both low and high resonant frequencies. Thus, the conventional double-tuned bird cage coils are dependently tuned such that more than one set of capacitances on the coil must be adjusted to tune either the lower or higher frequency. Variable capacitors are usually used to tune the resonant frequencies of conventional double-tuned bird cage coils, so significant amounts of time may be required for an iterative tuning procedure.
Thus, it is desired to develop a double-tuned bird cage coil that may be independently tuned to provide a substantially uniform magnetic field. Additionally, it is desired to develop a bird cage coil that minimizes the problems associated with solenoid inductors. Furthermore, it is desired to develop a multiple tuned bird cage coil to provide a substantially uniform magnetic field.